This invention relates generally to the field of semiconductor devices which include a heterojunction, such as a photovoltaic device.
Devices which rely on the presence of a heterojunction are well-known in the art. As used in this context, a heterojunction is usually formed by contact between a layer or region of one conductivity type with a layer or region of opposite conductivity (e.g., a “p-n” junction). Examples of these devices include thin film transistors, bipolar transistors, and photovoltaic devices (e.g., solar cells).
Photovoltaic devices convert radiation, such as solar, incandescent, or fluorescent radiation, into electrical energy. Sunlight is the typical source of radiation for most devices. The conversion to electrical energy is achieved by the well-known photovoltaic effect. According to this phenomenon, radiation striking a photovoltaic device is absorbed by an active region of the device, generating pairs of electrons and holes, which are sometimes collectively referred to as “photo-generated charge carriers.” The electrons and holes diffuse, and are collected at the contacts, often using an electric field built into the device.
The increasing interest in solar cells as a reliable form of clean, renewable energy has prompted great efforts in increasing the performance of the cells. One primary measurement for such performance is the energy conversion efficiency of the device. Conversion efficiency is usually measured as the amount of electrical energy generated by the device, as a proportion of the light energy which contacts its active surface. Even small increases in energy conversion efficiency, e.g., 1% or less, represent very significant advances in photovoltaic technology.
The performance of photovoltaic devices depends in large part on the composition and microstructure of each semiconductor layer. Specifically, crystalline semiconductor layers may introduce a number of undesirable defects to the device. For example, defect states which result from structural imperfections or impurity atoms may reside on the surface or within the bulk of monocrystalline semiconductor layers. Moreover, polycrystalline semiconductor materials may contain randomly-oriented grains, with grain boundaries which induce a large number of bulk and surface defect states.
The presence of various defects of this type can be the source of deleterious effects in the photovoltaic device. For example, many of the charge carriers recombine at the defect sites near the heterojunction, instead of continuing on their intended pathway to one or more collection electrodes. Thus, they become lost as current carriers. Recombination of the charge carriers is one of the chief reasons for decreased energy conversion efficiency.
The negative effects of surface defects can be minimized to some degree by passivation techniques. For example, a layer of intrinsic (i.e., undoped) semiconductor material can be formed on the surface of the crystalline substrate. The presence of this intrinsic layer decreases the recombination of charge carriers at the substrate surface, and thereby improves the performance of the photovoltaic device.
The concept of using this type of intrinsic layer is generally described in U.S. Pat. No. 5,213,628 (Noguchi et al). Noguchi describes a photovoltaic device which includes a monocrystalline or polycrystalline semiconductor layer of a selected conductivity type. A substantially intrinsic layer of 250 Angstroms or less is formed over the substrate. A substantially amorphous layer is formed over the intrinsic layer, having a conductivity opposite that of the substrate, and completing a “semiconductor sandwich structure”. The photovoltaic device is completed by the addition of a light-transparent electrode over the amorphous layer, and a back electrode attached to the underside of the substrate.
The photovoltaic devices described in the Noguchi patent may considerably minimize the problem of charge carrier recombination in some situations. For example, the presence of the intrinsic layer at selected thicknesses is said to increase the photoelectric conversion efficiency of the device. Moreover, the concept of passivating the surfaces of semiconductor substrates in this manner has been described in a number of references since the issuance of Noguchi et al. Examples include U.S. Pat. No. 5,648,675 (Terada et al); and U.S. Patent Publications 2002/0069911 A1 (Nakamura et al): 2003/0168660 A1 (Terakawa et al); and 2005/0062041 A1 (Terakawa et al).
While the references mentioned above address the recombination problem to some degree, there are some considerable drawbacks remaining. For example, the presence of the intrinsic layer, while beneficial to some extent, results in the formation of yet another interface, i.e., between the intrinsic layer and the overlying amorphous layer. This new interface is yet another site for impurities and spurious contaminants to become trapped and to accumulate, and possibly cause additional recombination of the charge carriers. For example, interruptions between the deposition steps during fabrication of a multilayer structure can provide unwelcome opportunities for the entry of the contaminants.
Moreover, abrupt band bending at the interface, due to an abrupt change in conductivity, and/or variations in band gap, can lead to a high density of interface states and energetically favorable sinks for holes and electrons, which is another possible source of recombination.
With some of these concerns in mind, improved photovoltaic devices would be welcome in the art. The devices should minimize the problem of charge-carrier recombination at various interface regions between semiconductor layers. Moreover, the devices should exhibit electrical properties which ensure good photovoltaic performance, e.g., energy conversion efficiency. Furthermore, the devices should be capable of being made efficiently and economically. The fabrication of the devices should reduce the deposition steps which would allow the entry of excessive levels of impurities and other defects.